/*---------------------------------------------------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Copyright (C) 2022-2025 OpenFOAM Foundation
     \\/     M anipulation  |
-------------------------------------------------------------------------------
License
    This file is part of OpenFOAM.

    OpenFOAM is free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    OpenFOAM is distributed in the hope that it will be useful, but WITHOUT
    ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
    FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    for more details.

    You should have received a copy of the GNU General Public License
    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.

\*---------------------------------------------------------------------------*/

#include "isothermalFluid.H"
#include "fvcFlux.H"
#include "fvcSmooth.H"
#include "fvcSurfaceIntegrate.H"

// * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //

void Foam::solvers::isothermalFluid::setRDeltaT()
{
    const volScalarField& psi = thermo.psi();

    volScalarField& rDeltaT = trDeltaT.ref();

    const dictionary& pimpleDict = pimple.dict();

    const scalar maxCo
    (
        pimpleDict.lookupOrDefault<scalar>("maxCo", 0.8)
    );

    const scalar rDeltaTSmoothingCoeff
    (
        pimpleDict.lookupOrDefault<scalar>("rDeltaTSmoothingCoeff", 0.02)
    );

    const volScalarField rDeltaT0("rDeltaT0", rDeltaT);

    // Set the reciprocal time-step from the local Courant number
    rDeltaT.internalFieldRef() =
        fvc::surfaceSum(mag(phi))/((2*maxCo)*mesh.V()*rho());

    // Set the reciprocal time-step from the local acoustic Courant number
    if (pimple.transonic())
    {
        surfaceScalarField phid("phid", fvc::interpolate(psi)*fvc::flux(U));

        rDeltaT.internalFieldRef() =
            max
            (
                rDeltaT(),
                fvc::surfaceSum(mag(phid))/((2*maxCo)*mesh.V()*psi())
            );
    }

    // Clip to user-defined maximum and minimum time-steps
    scalar minRDeltaT = gMin(rDeltaT.primitiveField());
    if (pimpleDict.found("maxDeltaT") || minRDeltaT < rootVSmall)
    {
        const scalar clipRDeltaT = 1/pimpleDict.lookup<scalar>("maxDeltaT");
        rDeltaT.max(clipRDeltaT);
        minRDeltaT = max(minRDeltaT, clipRDeltaT);
    }
    if (pimpleDict.found("minDeltaT"))
    {
        const scalar clipRDeltaT = 1/pimpleDict.lookup<scalar>("minDeltaT");
        rDeltaT.min(clipRDeltaT);
        minRDeltaT = min(minRDeltaT, clipRDeltaT);
    }

    Info<< "Flow time scale min/max = "
        << gMin(1/rDeltaT.primitiveField()) << ", " << 1/minRDeltaT << endl;

    // Update the boundary values of the reciprocal time-step
    rDeltaT.correctBoundaryConditions();

    if (rDeltaTSmoothingCoeff < 1.0)
    {
        fvc::smooth(rDeltaT, rDeltaTSmoothingCoeff);
    }

    Info<< "Smoothed flow time scale min/max = "
        << gMin(1/rDeltaT.primitiveField())
        << ", " << gMax(1/rDeltaT.primitiveField()) << endl;

    // Limit rate of change of time scale
    // - reduce as much as required
    // - only increase at a fraction of old time scale
    if
    (
        pimpleDict.found("rDeltaTDampingCoeff")
     && runTime.timeIndex() > runTime.startTimeIndex() + 1
    )
    {
        // Damping coefficient (1-0)
        const scalar rDeltaTDampingCoeff
        (
            pimpleDict.lookup<scalar>("rDeltaTDampingCoeff")
        );

        rDeltaT =
            rDeltaT0
           *max(rDeltaT/rDeltaT0, scalar(1) - rDeltaTDampingCoeff);

        Info<< "Damped flow time scale min/max = "
            << gMin(1/rDeltaT.primitiveField())
            << ", " << gMax(1/rDeltaT.primitiveField()) << endl;
    }
}


// ************************************************************************* //
